Crack Paths 2009

growth ceased before the crack length reached about l = 0.4 m m ,and the C G Rwas

restored and growth curves could be approximated as straight lines (l > 0.4 mm). Under

high stress (Fig. 2b), there was no significant retardation of crack growth. The initial

growth curve could be approximated as a slightly concave line, followed by a straight

line for a large crack length (l > 0.3 mm). That is, the C G Raccelerated with an increase

in the crack length.

1.0x106 2.0x106 3.0x106 1200MPa Number ofcycles N σa a

b

σa

0.010.1

0.0

2 40 0 M P a Numberof cycles N

0.010.101

0.0

1.0x105 2.0x105 3.0x105

Figure 2. Crack growth data: (a) σa = 100 and 120 MPa, (b) σa = 200 and 240 MPa.

To clarify the formation of surface damage, the change in surface states during

stressing was monitored by OM.Fig. 3 shows the formation process of surface damage

at σa = 120 MPa. It showed that primary SBs form at an early stage of cycling; in

particular, the morphology of SBs at a high stress amplitude (σa > 200 MPa) can be

classified into persistent slip band (PSB)-like SBs [15,17]. The number and area of the

damaged regions slowly increase with further cycling up to a specific cycle ratio,

depending on the stress amplitude; e.g. N/Nf ≈ 0.4 and 0.2 for σa = 120 and 240 MPa,

respectively. Once this specific cycle ratio is exceeded, both the number and area of the

damaged regions significantly rise. The damaged region formed after the specific cycle

ratio is reached is roughly classified as a secondary SB [20] and insular damage with a

complex morphological feature [17]. In addition, the change in surface Vickers hardness

(load: 2.9 N) during repeated stressing was studied. The surface hardness showed an

initial moderate reduction and subsequently a significant reduction. By investigating the

change in surface hardness and the formation of surface damage together, it was found

this considerably large drop in hardness is closely related to the significant formation of

damaged areas. Several studies on the GBs of U F G copper prepared by SPD have

suggested the existence of highly non-equilibrium GBs with high energy, excess

volume, and long-range stress fields, etc. [22,23], where the absorption of dislocations

into GBs takes place with remarkably enhanced diffusion [23]. The initial (gentle)

hardness drop appears to result mainly from a release of strain energy relating to

primary SB formation and a decrease in the density of dislocations inside the grains. A

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